Configuring pg_auto_failover

Several defaults settings of pg_auto_failover can be reviewed and changed depending on the trade-offs you want to implement in your own production setup. The settings that you can change will have an impact of the following operations:

  • Deciding when to promote the secondary

    pg_auto_failover decides to implement a failover to the secondary node when it detects that the primary node is unhealthy. Changing the following settings will have an impact on when the pg_auto_failover monitor decides to promote the secondary PostgreSQL node:

    pgautofailover.health_check_max_retries
    pgautofailover.health_check_period
    pgautofailover.health_check_retry_delay
    pgautofailover.health_check_timeout
    pgautofailover.node_considered_unhealthy_timeout
    
  • Time taken to promote the secondary

    At secondary promotion time, pg_auto_failover waits for the following timeout to make sure that all pending writes on the primary server made it to the secondary at shutdown time, thus preventing data loss.:

    pgautofailover.primary_demote_timeout
    
  • Preventing promotion of the secondary

    pg_auto_failover implements a trade-off where data availability trumps service availability. When the primary node of a PostgreSQL service is detected unhealthy, the secondary is only promoted if it was known to be eligible at the moment when the primary is lost.

    In the case when synchronous replication was in use at the moment when the primary node is lost, then we know we can switch to the secondary safely, and the wal lag is 0 in that case.

    In the case when the secondary server had been detected unhealthy before, then the pg_auto_failover monitor switches it from the sate SECONDARY to the state CATCHING-UP and promotion is prevented then.

    The following setting allows to still promote the secondary, allowing for a window of data loss:

    pgautofailover.promote_wal_log_threshold
    

pg_auto_failover Monitor

The configuration for the behavior of the monitor happens in the PostgreSQL database where the extension has been deployed:

pg_auto_failover=> select name, setting, unit, short_desc from pg_settings where name ~ 'pgautofailover.';                                                                                                                                                                                                                                                           -[ RECORD 1 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.enable_sync_wal_log_threshold
setting    | 16777216
unit       |
short_desc | Don't enable synchronous replication until secondary xlog is within this many bytes of the primary's
-[ RECORD 2 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.health_check_max_retries
setting    | 2
unit       |
short_desc | Maximum number of re-tries before marking a node as failed.
-[ RECORD 3 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.health_check_period
setting    | 20000
unit       | ms
short_desc | Duration between each check (in milliseconds).
-[ RECORD 4 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.health_check_retry_delay
setting    | 2000
unit       | ms
short_desc | Delay between consecutive retries.
-[ RECORD 5 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.health_check_timeout
setting    | 5000
unit       | ms
short_desc | Connect timeout (in milliseconds).
-[ RECORD 6 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.node_considered_unhealthy_timeout
setting    | 20000
unit       | ms
short_desc | Mark node unhealthy if last ping was over this long ago
-[ RECORD 7 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.primary_demote_timeout
setting    | 30000
unit       | ms
short_desc | Give the primary this long to drain before promoting the secondary
-[ RECORD 8 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.promote_wal_log_threshold
setting    | 16777216
unit       |
short_desc | Don't promote secondary unless xlog is with this many bytes of the master
-[ RECORD 9 ]----------------------------------------------------------------------------------------------------
name       | pgautofailover.startup_grace_period
setting    | 10000
unit       | ms
short_desc | Wait for at least this much time after startup before initiating a failover.

You can edit the parameters as usual with PostgreSQL, either in the postgresql.conf file or using ALTER DATABASE pg_auto_failover SET parameter = value; commands, then issuing a reload.

pg_auto_failover Keeper Service

For an introduction to the pg_autoctl commands relevant to the pg_auto_failover Keeper configuration, please see pg_autoctl configuration and state files.

An example configuration file looks like the following:

[pg_autoctl]
role = keeper
monitor = postgres://autoctl_node@192.168.1.34:6000/pg_auto_failover
formation = default
group = 0
nodename = node1.db
nodekind = standalone

[postgresql]
pgdata = /data/pgsql/
pg_ctl = /usr/pgsql-10/bin/pg_ctl
dbname = postgres
host = /tmp
port = 5000

[replication]
slot = pgautofailover_standby
maximum_backup_rate = 100M
backup_directory = /data/backup/node1.db

[timeout]
network_partition_timeout = 20
postgresql_restart_failure_timeout = 20
postgresql_restart_failure_max_retries = 3

To output, edit and check entries of the configuration, the following commands are provided:

pg_autoctl config check [--pgdata <pgdata>]
pg_autoctl config get [--pgdata <pgdata>] section.option
pg_autoctl config set [--pgdata <pgdata>] section.option value

The [postgresql] section is discovered automatically by the pg_autoctl command and is not intended to be changed manually.

pg_autoctl.monitor

PostgreSQL service URL of the pg_auto_failover monitor, as given in the output of the pg_autoctl show uri command.

pg_autoctl.formation

A single pg_auto_failover monitor may handle several postgres formations. The default formation name default is usually fine.

pg_autoctl.group

This information is retrieved by the pg_auto_failover keeper when registering a node to the monitor, and should not be changed afterwards. Use at your own risk.

pg_autoctl.nodename

Node hostname used by all the other nodes in the cluster to contact this node. In particular, if this node is a primary then its standby uses that address to setup streaming replication.

replication.slot

Name of the PostgreSQL replication slot used in the streaming replication setup automatically deployed by pg_auto_failover. Replication slots can’t be renamed in PostgreSQL.

replication.maximum_backup_rate

When pg_auto_failover (re-)builds a standby node using the pg_basebackup command, this parameter is given to pg_basebackup to throttle the network bandwidth used. Defaults to 100Mbps.

replication.backup_directory

When pg_auto_failover (re-)builds a standby node using the pg_basebackup command, this parameter is the target directory where to copy the bits from the primary server. When the copy has been successful, then the directory is renamed to postgresql.pgdata.

The default value is computed from ${PGDATA}/../backup/${nodename} and can be set to any value of your preference. Remember that the directory renaming is an atomic operation only when both the source and the target of the copy are in the same filesystem, at least in Unix systems.

timeout

This section allows to setup the behavior of the pg_auto_failover keeper in interesting scenarios.

timeout.network_partition_timeout

Timeout in seconds before we consider failure to communicate with other nodes indicates a network partition. This check is only done on a PRIMARY server, so other nodes mean both the monitor and the standby.

When a PRIMARY node is detected to be on the losing side of a network partition, the pg_auto_failover keeper enters the DEMOTE state and stops the PostgreSQL instance in order to protect against split brain situations.

The default is 20s.

timeout.postgresql_restart_failure_timeout

timeout.postgresql_restart_failure_max_retries

When PostgreSQL is not running, the first thing the pg_auto_failover keeper does is try to restart it. In case of a transient failure (e.g. file system is full, or other dynamic OS resource constraint), the best course of action is to try again for a little while before reaching out to the monitor and ask for a failover.

The pg_auto_failover keeper tries to restart PostgreSQL timeout.postgresql_restart_failure_max_retries times in a row (default 3) or up to timeout.postgresql_restart_failure_timeout (defaults 20s) since it detected that PostgreSQL is not running, whichever comes first.